I would try putting the IP address of your Raspberry Pi in to the address field. I’m not entirely sure what effect is has on the Magic Mirror process, since the web server should listen on a port as opposed to a particular IP. I do know that I’ve had success with putting the IP address there and accessing my mirrors via web browser. You can find your Pi’s IP address by typing ip address at the command prompt on the Pi. Assuming that you’re using the WiFi connection to connect to your network, look for the entry wlan0.
If that still doesn’t work, I have some further troubleshooting ideas for you. For starters, try viewing your mirror’s interface while you’re at home. Grab your laptop and point its browser at your mirror’s IP address & port. If that works, then at least you know the Pi is serving up pages to you. Try disconnecting from your home network, firing up your VPN with a shared cellular connection (or any other connection that is not your home network) and try it again.
Restarting in server mode will only prevent the Magic Mirror interface from using electron to display the mirror via the HDMI jack on the Pi. The Magic Mirror framework is essentially a hosted website. Electron is used to display that website on the local display.
However: Any changes you make to the config.js file will require you to restart the Magic Mirror process for the changes to take effect.